Fibronectin (FN) has been shown to be a major nonspecific opsonin in plasma and to be depleted in severely ill patients. In a small, uncontrolled study, restoration of fibronectin by giving cryoprecipitate had a beneficial effect on surgical patients with sepsis. Preliminary work in our laboratory has demonstrated that FN is an effective opsonin for S. aureus ATCC 25923. The goals for this project are (1) to characterize the opsonic properties of fibronectin for multiple bacterial strains, (2) to determine which step(s) FN augments in neutrophil killing of bacteria, (3) to isolate and identify the FN binding substance in S. aureus, and (4) to characterize the role of FN in bacterial adherence to human cells in tissue culture. The activity of FN will be assayed by an in vitro neutrophil bactericidal assay and by clumping assays. Clinical isolates and selected type culture strains of bacteria will be assayed. By the use of lysostaphin and tritiated labelled S. aures ATCC 25923, attachment, phagocytosis, and digestion of bacteria by human neutrophils will be differentiated. S. aureus will be fractionated by utilizing combinations of detergents (e.g., SDS), enzymes and acids. The fraction which binds to FN will be further characterized. Also, ultrastructural analysis of bacteria-FN-neutrophil interactions will be performed. Fibronectin is not only a serum protein, but also a major component of the interstial stroma. Thus, although circulating FN may function as an opsonin, tissue FN may promote bacterial adherence to tissue stroma. FN is laid down on the basement membrane surface, but not on the luminal surface of endothelium. Either a break in the endothelium or a clot, which would include FN, on the endothelial surface could act as an attachment site for bacteria. We anticipate that these studies will lead to a greater understanding of the nonspecific immune system-and perhaps allow for more rational administration of FN as therapy for critically ill patients. Moreover, if FN proves to serve as a bacterial attachment site on damaged endothelium, then this knowledge might lead to the development of methods which could block adherence and thus prevent endocarditis.